BMEN20002Anatomy &Physiology forBioengineering

A Prof Hamish Meffin

hmeffin@unimelb.edu.au

Department of Biomedical Engineering

Entrance to the Biosciences building surrounded by ivy

Lecture 6.3: Auditory System

Overview

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Semicircularducts

Vestibular nerve

Cochlear nerve

Vestibulocochlearnerve (VIII)

Internal

auditory meatus

Cochlea

Figure 14.34

The Ear

Pinna

External auditory

meatus

Tympanicmembrane

Malleus

Incus

Stapes

Outer ear

Middle ear

Inner ear

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Ear Anatomy & Physiology

Outer Ear(pinna and ear canal): Has air medium

Serves to direct and transmit sound.

TympanyMembrane

Transduces air vibration into mechanical vibration and amplifies the intensity ofthe vibration.

Middle Ear(malleus, Incus, Stapes)

If there is no impedance balancing of the air medium from the outer ear and theliquid medium of the inner ear, only ~0.1% of the incident power of the soundwaves would be transmitted (i.e., an attenuation of 30 dB).

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Ear Physiology

Transduction of sound into an auditory perception

Sound is a propagating pressure wave.

Perception of sound involves the electrical activity of neurons in the auditorycortex of the brain.

The transduction process is the means by which the pressure waves in air (amechanical stimulus) is converted into neural activity (action potentials).

This process involves a number of stages, some of which involve conductionand impedance matching.

The path of sound

ear canalvibrate tympanic membrane

vibrateossicles(3 bones: Malleus, Incus, Stapes)

vibrate oval window of cochlea

create waves in cochlea fluid

create standing waves in basilar membrane

movement of hair cells generates electrical activity through mechanicallygated ionic channels

hair cells stimulate the auditory nerve

series of action potentials up to the auditory cortex.

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cochlea_mensch_1a
cochlea_mensch_1b

Basilar membrane travelling wave

bmtravellingwave
fig_ee32

Cochlear anatomy

Kiang et al., 1974

AN

CNS

cochlea

ear_oview_bwphoto

Cochlear duct

cochlea-overview

Cochlear Duct

Organ ofCorti

Stereocillia-SEM
ocorti-overview

Organ of Corti

Organ ofCorti: The transducer

Vibration of the basilar membrane produces shear forces thatbend the stereocilia (hairs protruding from the hair cells)against the tectorial membrane

Movement of the stereocilia either cause the hair cell todepolarise or hyperpolarise, depending upon the direction ofmovement

Changes in the membrane potential of the hair cell generate an APin the nerve fibre attached to the hair cell.

Electron Micrograph: Apparent are inner hair cell and outer haircell and rows ofstereocillia

Hair Cell Innervation

spoendlin_sgc

Scala tympani

Rosenthal’s

canal

Spoendlin, 1984

Hair Cell Innervation

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Inner Hair Cell photo + diagram
Inner Hair Cell

Inner Hair Cells

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Resting Electrical Potentials of the Cochlear

Two classes of potential: Resting potentials and stimulus-related potentials

Maintenance of large DC potentials between endolymphatic andperilymphaticspaces plays an important role in active cochlear tuning

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Inner Hair Cell Receptor Potential

Receptor Potential

Below 1000 Hzfollows oscillations

Above 3500 Hzfollow envelope only

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Cochlea mechanics: Active processes

active-tip
active-loop

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Active Cochlea Mechanics

Auditory Nerve

spoendlin_sgc

Scala tympani

Rosenthal’s

canal

Spoendlin, 1984

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fig_ee34

Frequency Tuning Curve

log frequency

100

80

60

40

20

Characteristic

Frequency

Threshold

Bandwidth

log amplitude (dB SPL)

Frequency Tuning Curve

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SPIKES

The discharges of cochlear nerve fibres to low-frequency sounds arenot random;

they occur at particular times (phase locking).

Evans (1975)

Temporal Frequency Coding

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Temporal Coding

Information is contained in the relative timing of individual action potentials.E.g.

The temporal code contains detailed information about the phase of a periodicsignal, i.e. much more fine-grained information about the stimulus down to aresolution of ~10 us at a population level.

Note: This information is in addition to rate information, not in contradiction ofit.

Temporal information is observed in the auditory pathway, but diminishes inresolution from auditory nerve to auditory cortex (~10 us to ~10 ms)

Does temporal information play a functional role or is it an epiphenomenon?

What evidence is there that the temporal code is used by the brain?

Summary

Outer Ear: amplification and direction filtering.

Middle Ear: impedance matching.

Cochlear & Basilar Membrane: spectral analyser.

Organ ofCorti: Acoustic to neuro-electric transducer.

Inner Hair Cells: Synapse type I AN fibres projectingcentrally.

Outer Hair Cells: Amplification and improved frequencytuning.

Auditory Nerve: Carries signal to the auditory braincentres. Information is convey by both rate and timing ofaction potentials.

The Auditory Pathway

Brainstem & Midbrain

BrainStem2

Dorsal

Ventral

Rostral

Caudal

Auditory Pathway

Figure from MIT OpenCourseWare

lateral

rostral

dorsal

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Octopus

Spherical bushy

Globular bushy

Fusiform

Stellate

Stellate

Ventral

Anterior

Posterior

Dorsal

DCN

AVCN

PVCN

AN

Cochlear Nucleus

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Parallel pathways emerge from the CN

VCN extracts and enhances timing and spectrum, as well as information on thewaveform envelope (e.g., amplitude modulation)

Bushy cellsTBSOCDNLLIClocalisation“where”

T-stellate cellTBVNLLICspectrum, AM“what”

Octopus cellVNLLICtemporal pattern

DCNIClocalisation & spectrum

monaural pinna (spectral) cues

These parallel streams converge in the central nucleus of the inferior colliculus (IC)in the midbrain

Abbreviations:

TBTrapezoid body

SOCsuperior olivary complex

DNLLdorsal nucleus of the lateral lemniscus

VNLLventral nucleus of the lateral lemniscus

ICinterior colliculus

Brainstem & Thalamus

BrainStem2

Dorsal

Ventral

Rostral

Caudal

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Auditory Cortex

wernicke's area

from http://www.physiology.wisc.edu/neuro524/audition02-524.htm

Left hemisphere is specialised in the analysis, perception and production oflanguage.

Right hemisphere implicated in the perception of duration, intensity,intonation, melody.

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cortical layers from FN CH 22 fig 8

Laminar Structure

From: Fundamental Neuroscience (2002), Elsevier, Fig 8, CH 22

Supragranular

layers (1-3)

Granular layer (4)

Infragranular

layers (5-6)

stellate cell

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Structure of Cerebral Cortex

Columnar Organisation

Smith & Populin, 1977

Columnar Organisation

Both anatomical and electrophysiological evidence supports acolumnar organization of auditory cortex.

Neurons in vertical electrode penetrations have similar responseproperties. Eg. CF, binaural response, bandwidth.

Synaptic connections between neurons with similar responseproperties

Cortical Maps

Binaural interaction map

Spectral integration map

Read et al., 2002

Smith & Populin, 1977

Cortical Maps

Spatial clustered or topographic arrangement of neural responseacross the cortical surface.

Cortical maps of:

Frequency (tonotopic)

Binaural response type

Bandwidth.

Binaural and bandwidth maps are roughly orthogonal to tonotopicmap.

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Summary of the Auditory Pathway

Tonotopic organisation